Vapor electric device



Jan. 18, 1949. w. E. PAKALA VAPOR ELECTRIC DEVICE Filed March 20, 194? 4 Sheets-Sheet 1 INVENTOR WITNESSES: awyz 2w. 6.

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BY Jt/f. 54m ATTORNEY Jan. 18, 1949.

Filed March 20 w. E. PAKALA 2,459,582

VAPOR ELECTRIC DEVICE 4 Sheets-Sheet 2 WlTN ESSES:

dvyz 22 INVENTOR M/fiam EPaKa/a.

ATTORN EY Jan. 18, 1949. w PAKALA 2,459,582

VAPOR ELECTRIC DEVICE Filed March 20, 1947 4 Sheets-Sheet 3 WITNESSES: I h INVENTOR MYfid/TJEQZAa/a,

BY flu 9% ATTORNEY Jan. 18, 1949. w. E. PAKALA 2,459,582

VAPOR ELECTRIC DEVICE Filed March 20, 1947 4 Sheets-Sheet 4 WITNESSES:

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INVENTOR /3 ATTORNEY Patented Jan. 18, 1949 VAPOR ELECTRIC DEVICE William E. Pakala, Forest Hills, Pa., assignor to Westinghouse Electric Corporation, EastPittsburgh, Pa., a corporation of Pennsylvania Application March 20, 1947,, Serial No. 735,991

19 Claims.

My invention relates to a vapor electric device, and particularly to a control circuit using a single controlled electric valve for applying periodic excitation to two alternately conducting vapor electric valves.

This is a continuation-in-part of my application Serial No. 531,096, filed April 14, 1944, now abandoned, and assigned to the same assignee as this application.

In the construction of vapor electric devices of the make-alive type, the equipment for producing the excitation impulses is of substantially uniform size regardless of the power rating of the rectifier. It thus frequently happens that for small power ratings the control equipment occupies more space than the rectifier per se. Also, it has heretofore been necessary to utilize a control valve for each valve of the main rectifier, thus increasing the cost and size of the excitation equipment.

According to my invention, I materially reduce the size and space requirements of the excitation system by utilizing a single controlled electric valve to apply excitation impulses to two alternately conducting vapor electric valves.

In the construction according to my invention, a capacitor is arranged in charging relation to a suitable source of control energy and alternately discharged through the make-alive electrodes of a pair of valves by means of a control valve in a common return circuit from the load device to the impulsing capacitor.

It is accordingly an object of my invention to provide a control circuit utilizing a single control valve to apply excitation impulses to a plurality of vapor electric devices.

It is a further object of my inventionto provide a control system having a minimum number of parts.

It is a further object of my invention to provide a control system capable of being installed in the minimum of space.

It is a further object of my invention to utilize a single control valve to control consecutive impulses of opposite polarity.

It is'a further object of my invention to provide means responsive to the available control energy to trigger the control valve.

Other-objects and advantages of my invention will be apparent from the following detailed description, taken in conjunction with the accompanying drawings, in which:

Fig. 1 is a schematic illustration of a vapor electric converter according to my invention;

Fig. 2 is a modification similar to Fig. 1 showing the utilization of a mercury-pool type control tube;

Fig. 3 is a diagrammatic illustration of the voltageand current characteristics of the control system according to my invention;

Fig, 4 is a further modification showing the utilization of a winding to complete the return circuit of the capacitor;

Fig. 5 is a further modification showing the utilization of high voltage in the main portion of the excitation circuit;

Fig. 6 is a modification similar to Fig. 1 showing a direct connection of the excitation circuit to the anode terminals of the converter;

Fig 7 is a schematic illustration showing the adaptation of my system to a" conversion circuit utilizing a plurality of pairs of alternately con ducting valves;

Fig. 8 is a schematic illustration of a typical embodiment of a conversion system utilizing my invention; and p v Fig. 9 is a diagrammatic illustration of the voltages applied to the grid of a control tube as utilized in Fig. 8.

In the illustrated embodiment of my invention according to Figs. 1 and 8, an alternating-current supply circuit I0 is connected to a load circuit II' by means of a converter transformer l2 and the flow of current between the circuits l0 and H is controlled by means of a pair of alternately conducting valves l3 and M of the makealive type, each of said valves l3 and 14 comprising an anode l5 connected to a corresponding terminal of the rectifier transformer l2, a vaporizable reconstructing cathode l6 of suitable material, such as mercury, placed in spaced insulating relation to the anode l5, and an excitation electrode l'l, preferably of the make-alive type, comprising a pointed, rod-like member of high resistance material in substantially permanent contact with the pool of cathode material.

It is usually desirable to provide suitable switching mechanism 52 between the alternating-current circuit Ill and the rectifier transformer l2, and also to provide switching mecha- 3 nism I2 between the converter and the load circuit II'.

The excitation system comprises a suitable source of alternating current having a frequency corresponding to the frequency applied to the converter valves I3-I4. I have herein shown the control potential as being supplied through switch 3| from the alternating-current circuit I ll, preferably through a suitable phase-shifting device to the primary winding of a transformer 2i which is connected in charging relation to an excitation capaJGit'ortZE. The -;phase shifter {20 may be entirely omittedwhenon'l'y a moderate phase shift is desired, thus further reducing the size and weight of the control cabinet. Preferaactor, is provided between the charging transformer 2i and the capacitori22, not onlyvtocontrol the rate of charge of the capacitorf22 but to control the follow current from the charging toward itsrespective :capaoitorterminal 'so that current can flow-- ;from the-conductor 32 to either terminal of the capacitortug-depending upon the polarity ,rof' the capacitor charge.

The connection *24 contains a unidirectional conductor 2'! while connectionl-2.5-contains.-aunidirectional tconduotor 28. The unidirectional conductorszZI andr28 are sorpoled-that when one terminal of :the. capacitor '22 positive, current will flow throughunidirectional conductor 21, connection-.24, lmake-aliveielectrode Il' -and cathode .I6 =-.of valve I13 4 10 )theacathode-terminal 26 thence through connection 32 and unidirectional conductor :30 .tothe .negative .lterminal ..of the capacitor.22.;l-then. onithelalternate polarity, .thecurrent flows through unidirectional;conductor.28, connection-r25,,the-make-aliveIII and cathode: I E of valveiIAtto the-v cathode terminal .26 thence through connection 32 and unidirectional iconductor ,29 .to the negative. terminal of'rcapacitor 22. .It .will..be,-s.een that the unidirectionalcom ductors (Nine-:29? thus .providea bridgetyperecti-fierlacross.theterminals ofthe capacitor 22 Withlllhe unidirectional current-carrying connection .32 iacrossvthe unidirectional terminals .26 andt33. 1

..'JIhe-,make-alivelelectrodes .II of alternately conducting valves $1.5 are .conrrectcd in .seriescin cuit relationHtonnidirectional :conductors .2! and ZBJrespectiVely intalternatearms of .thebridgecircuit. The makervalivetelectrodes thusalternately provide the load .onlthe ,system. Obviously, ,inad of .direct y couplin .the lmakeealive electrodesthey may beindirectly couplediaslbylmeans ofthetransformenAB. I

..In .order =to control the .fiowiof .currentin .the loadudevice -.I.1,.a controllable :electric valve 35 is connected the unidirectional current-carrying connection 32. Preferably, the valve .35 .is of .the gaseous .gri'd wconltrnlled variety and may have a hot cathode as shown in Fig, 1, or a pool-type bly an impedance 23, usually in the formofia rethe converter.- The deviceewill then ;be put in operation by controlling :the gridl36 o f the controlled valve 35 in the return circuit 32 to permit discharge of the capacitor 22 through the makealive electrode I1, the common connection 26, the controlled electric valve 35 and, if necessary,

through tcurrent-limiting resistor and unidirec- 'tiorial conductor .30 to the negative side of the capacitor 22. i

As will be seen from the diagrammatic illustration of Fig; 3, the voltage 8c of the capacitor 22 is adjusted so as to have sufiicient potential derived from the supply potential on at the desired firin instant .to produce a current impulse 40 ,havingsuliicient magnitude tocreate .a. cathode spot in thewconverter. Since there will be ;a substantial component of potential r'applied through the charging impedance 23 and the unidirectional conductor 21 tothemake-alive.elec-' trode .IJI,.current M will continue to'lfiow until such time as the voltage ec drops to a potential which will fail to produce current flow through the impedance of the circuit. .The excitation impulse'ct therefore has a steep wave front ofhigh amplitudefor ashort duration and a. follow cur rent AI oftrailing Waves'hape'for the continuation of the excitation cycle.

The modification accordingto Fig. 2-is identical with that 'of Fig. ,1; 'eXcept that a mercury-pool control valve :35{ has been substituted for the hot cathode controlvalvef3'5 'of Fig. 1. Astafting electrode 39 'preferablyof the make-alive type isprovid-edand-th"system is put into operation by closing ,a starting switch which permits the excitation impulse 40 to initiate a-cathdde spot in the "control valve? 5 which is then maintained by 'means of akeep-alive arc from any suitable source '34, while the grid 36 is Controlled to control the actual beginning-of current impulses through the alternately "conducting valves I3- I 4 ofthe converter. g

In the modification according to Figs. 4 and'], the return circuit 32, is completed by meansof a winding 45 ofgrelativelyhigh impedance such as atrans'former winding connectedacross the terminals of the excitationcapacitor"-22, and the return circuit 32 is connected, to an'intermediate tap'46 .inthe high impedance winding 45.' Fig. 5 discloses a further rnodification particularly adapted to thegutilizationof high potential control devices; Thesupply-transformer 2i" may be either a-double windingor' a singlewinding transformer, the capacitor 22 being charged to 'high' potentials byconn'ection tothe secondary or high potential terminals of the transformer .2 I The discharge circuit isthen completed by means of a firing transformer 48, the primary 39 of which isalternately energized from the excitation capacitoriZ'Z and the current is then stepped up or rather the voltage is reduced to the necessary potential foroperating the make-alive electrodes Ii' pbymeans ofsecondary windings. 5il+5l energize'd'by the'current impulse in the'firing transformer '48. In 'orderto prevent reverse current through the make-alive electrodes H, a unidirectional conductor 53 or 54 is placed in series with each of, the make-alive electrodes l1. Therefore, the secondary windings 50-5] will alternately carry current, depending upon the polarity impressed on the firing transformer 48. l

The modification of 1 Fig. 6 is substantially similar to the modification of Fig; 1 except that the charging transformer 2| and its associated phase shifter has been omitted, and the impulsing capacitor 22 directly connected across the terminals of the rectifier transformer l2. For purposes of balance, I prefer to split the charging impedance 23 and to place a section 23' of this impedance on each side of the capacitor 22. The operation is otherwise identical with the operation as described according to Fig. 1.

Fig. ,7 illustrates the method of employing my control system of Fig. 4 for controlling a plurality of pairs of converter valves as is usually done in commercial circuits. The polyphase circuit I0 is connected to the direct-current circuit H by means of a multiplied phase transformer l2 herein illustrated as of the double three-phase variety and the resultant pairs of valves are each controlled by their individual impulsing circuit. The grid potentials of the control tubes may readily be controlled by a variable resistor in the primary circuit of a transformer 6| which supplies through the rectifying devices 62 a grid potential to the grids of the control tubes 35. The variable resistance 60 may be controlled in response to any desired operating condition and is herein shown as being controlled in response to the terminal voltage of the direct-current circuit II.

To further increase the utility of the control system, it is frequentlydesirable to have a completely self-contained automatic control of the valves 35 responsive to the state of the charge on the capacitor 22 and to control the phase relation of the charging potential applied to the capacitors 22 to regulate some characteristic of the converter. I have found that instead of utilizing a phase shifting device20 and an impedance 23, space and weight may be saved by utilizing phase shifting reactors which satisfactorily perform both the phase shifting and current control functions for a reasonably wide range of control. The phase controlling reactors 65 comprise a threelegged reactor core 66, on the outer legs of which are the windings 61, and on the middle leg a winding 88 supplied withdirect current from some suitable source such as the full-wave rectifier 69, the direct-current output of the rectifier 69 being varied in proportion to some value of the load which it is desired to regulate by some responsive device such as the silverstat 60 responsive to some control characteristic such as the output voltage of the converter. The phase control of the charging potential is obtained by varying the amount of current flowing in the windings 68 and so varying the reactance of the reactors 65.

Automatic control of the firing time of the valves 35 is obtained by means of a negative bias obtained from a full-wave rectifier 15 supplied from a voltage divider 16 connected across the capacitor 22. While several types of voltage dividers 16 may be used, I prefer to use a simple wound resistor as they are standard articles of manufacture and occupy a minimum of space.

The output of the rectifier 15 is stabilized by a capacitor 11 and a voltage I36 (see Fig. 9) is taken from a portion of a resistor 18 connected across the output of the rectifier l5 and applied between grid 36 of tube 35 and thepositive terminal 19 of rectifier l5.

A variable positive impulse is supplied to grid 36 by means of a voltage divider connected across the capacitor 22 and an impedance connected between the voltage divider and the junction 33 between rectifiers 29 and 30. Preferably, the voltage divider connected across the capacitor 22 comprises a resistor divided into two parts 8| and 82 with a mid-tap 83. Likewise, the impedance 3!! may be a simple commercial resistor.

The operation of the system is as follows: assuming potential applied to the excitation system so that positive potential flows through resistor section EH, impedance 80 and rectifier 30 to the capacitor 22, the potential between points 19 and 33 will be proportional to the potential of capacitor 22. The portion of impedance 80 included between 19 and 33 is so selected that the voltage i8!) rises toa value sufficient to trip the grid 36 at or near the maximum charge on the capacitor 22. The discharge of capacitor ;22 re duces the voltage I8!) to a value where potential I35 again makes grid 36 negative and care must be taken that any follow charge on the capacitor 22 does not increase I80. to a value which might produce false operation of valve 35. On the alternate half-cycle, the positive potential flows through 8283--29 and since the current in impedance 80 is unidirectional, the second peak is also positive and trips the grid to fire the alternate igniter.

An impedance 85 is usually inserted in series with valve 35 in order to control the discharge impulses of the capacitor 22, and a capacitor 86 is shunted between the grid 35 and the cathode of valve 35 to prevent false operation by any stray or transient impulses.

In a practical embodiment of the control system of Fig. 8 with capacitors 22 of 8 to 16 microiarads, the phase voltage of transformer 2|" was of the order of 300 to 50;) volts, the impedances BI and 82 were commercial wound resistors of 10,000 ohms each, while impedance 8?) was a simi lar resistor of 3000 ohms with a sliding contact and impedance B5 was of 5 ohms. It is apparent that these valves are dependent on the characteristic of valve 35 and the electrodes [1.

While for purposes of illustration, I have shown and described several specific embodiments of my invention, it will be apparent that changes and modifications can be made therein without departing from the true spirit of my invention or the scope of the appended claims.

I claim as my invention:

1. In an electric current translating system, for transferring electric energy between two dissimilar electric circuits, having at least a pair of alternately conducting electric valves, a control system comprisin a capacitor, circuits for charging said capacitor alternately in opposite polarity, an exciting electrode in each of said valves, a discharge circuit connecting said exciting electrodes to opposite terminals of said capacitor, a common connection between said exciting electrodes, p0- larity responsive circuit means connected to both terminals of said capacitor, a return circuit from said common connection to an intermediate point of said polarity responsive circuit means, a controlled electric valve in saidreturn circuitand a polarity selective device connected in series circuit relation between each of said exciting electrodes and said capacitor.

2. An electric current translating system for the alternate" terminals or said capacitor, a re t'urric'ircuit from a point intermediate said excitif electrodes to said intermediate connection of the circuit means, agrid controlled mercury cathode electric val've in said return circuit and polarity responsive means connected in series circuit'r uuen to'ach of said exciting electrodes.

3. An electric current conversion system 00111- prising at least a pair of alternately conducting valves each of said valves includin a main anode,

a vaporizalole cathode and a make-alive electrode, a source ofalternating control potential, a capacitor connected to he charged from said source, connections from the'opposite sides of the capacitor to the respective make-alike electrodes, unidirectional conductcr s in each of said connections, a pair of oppositely disposed unidirectional conduct'ors connected across said capacitor, a unidirectional returncircuit from the cathodes of said valves to a point intermediate said oppositely disposed unidirectional conductors and a controlled electric valve in said return circuit.

4. An electric current conversion system comprising'at least a pair of alternately conducting valves each of said valves including a main anode, a vaporizable cathode and a make-alive electrode, a source of alternating control potential, a capacitor connected to be charged from said source, connections from the opposite sides of the capacitor to the respective make-alive electrodes, polarity responsive means for controlling current flow in said connections, oppositely dis-- posed unidirectional conducting devices connected across said capacitor, a connection from the oathodes of said valves to a point intermediate said unidirectional conductors and a controlled electric valve in said connection.

5. An electric current conversion system comprising a pair of alternately conducting electric valves, means for impressing potential across said valves, ane'iiciting electrodein each 6i" said valves, a source of alternating control potential, capacitor means connected to be charged from said source of control potential, a discharge circuit for said capacitor means, including circuit means for connecting said exciting electrodes to opposite terminals of said capacitor means, a unidirectional conductor in series circuit relation with each of said exciting electrodes, at common turn connection from said exciting electrodes, oppositely disposed polarity responsive circuit means connected between the return circuit and the opposed terminals of said capacitor means, a grid controlled discharge device in said common return connection.

6. An electric current conversion system comprising a pair of alternately conducting electric valves, means for impressing potential across said valves, an exciting electrode in each of said valves, a source of alternating control potential, capacitor means connected to be charged from said source orcontrol pste'ntisi, adischarge circuit'for said capacitor means" including circuitmeans for connecting said exciting electrodes to opposite terminals of said capacitor means, a unidirec tional conductor in series circuit" relation With each of said exciting electrodes, a'cominon return connection from said exciting electrodes, opposed unidirectional conductors selectively connecting the return circuit in ccnductive'relation to the opposed terminals of said capacitor me'ans, a 'gr id controlled discharge device in said common return' connect1cn-,-ane means" responsive to the potential of said capacitor for supplying control potential to the rid or said gridcontrolled valve,

7. An electric conversion system comprising' at least a pair of alternately conducting vapor e'lejc' tri'c valves, each of said valves including a main anode, a vaporizable reconstructing cathode, and a make-aliveelectrodein contact with said oath ode,- a cap it or, ans for cyclically charging said capacitor inoppo'site polarity, circuit means including a common cathode connection for con necting said m ak ali ve electrodes across said cap acitoi a unidirectional conductor in series; With each of said make-hive electrodes, oppositely an: posed unidirectional conductors connected across said capacitor, a return circuit from saidcommon cathode connection tcapoint intermediate said oppositely disposed unidirectional conductors and a controlled electric valve in said return circuit;

8. An electric convers'ion sysftem comprisingat least a pair of alternately conducting vapor elec tricvalves, each of said valves including amen anode, a vap orizablereconstructing cathode, and a make-alivcelectrodeincontact with said'cath ode, a capacitor, means for cyclically charging said capacitor in opposite polarity, circuit mean including a common cathode connection for con-- necting said make-alive electrodes across said capacitor, a unidirectional conductor 7, in series with each of said make-alive electrodes, oppos'ite'ly disposed unidirectional conductors connected across said capacitcna return circuit from said common cathode connection to a point interme'diate said oppositely disposedunidirectional conductors and a controlled electric valve in said return circuit, and means to control said controlled electric valve to periodically establish a cathode spot in alternate ones of said valves.

9. In an electric conversion system having a pair of alternately conducting electric valves, a control system comprising" an exciting electrode for each of said valves, a source of alternating control potential, a capacitor connected to a said source to be charged therefrom during each halfcycle of the source potential, a bridge-type rectifier connected across the terminalsof the capacitor with the exciting electrodes connected in series circuit relation in alternately conducting arms of the bridge connected rectifier and grid controlled discharge device connected across the unidirectional terminals ofsaid bridge-type rectiher.

10. In an electric conversion system having'a pair of alternately conducting electric valves, a control system comprising an exciting electrode for each of said valves,- a source of alternating control potential, a capacitor connected to said source to he charged therefrom during each halfcycle of the source potential, connections from the opposite sides of said capacitor to the respective excitation electrodes, unidirectional conductors in each of said connections, a return circuit from said excitation; electrodes, impedance means connecting s'aidireturn circuit to both sides of said capacitor and a grid controlled electrlcvalve in said return connection.

"11. An electric current conversion system for interconnecting an alternating-current circuit anda direct-current circuit comprising a pair of alternately conducting valves, each of said valves includinga main anode, a mercury cathode and anexciting electrode associated with said cathode, a capacitor, means for charging said capacitor in oppositepolarity for succeeding half-cycles of the potential applied to said valves, a winding connected across said capacitor,connections including a common cathode connection for said valves connecting said exciting electrodes to opposed terminals of said winding, polarity responsive means in series with each of said exciting electrodes, a return connection from said common cathode connection to an intermediate tap in said winding and a controlled electric valve in said return connection.

12. An electric current conversion system for interconnecting an alternating-current circuit and a direct-current circuit comprising a pair of alternately conducting valves, each of said valves including a main anode, a mercury cathode and an exciting electrode associated with said cathode, a capacitor, means for charging said capacitor in opposite polarity for succeeding half-cycles of the potential applied to said valves, a winding connected across said capacitor, connections including a common cathode connection for said valves connecting said exciting electrodes to opposite ends of said winding, a return connection from said common cathode connection to an intermediate tap in said winding, a controlled electric valve in said return connection, and a unidirectional conductor in series with each of said exciting electrodes.

13. In an electric conversion system having at least a pair of alternately conducting electric valves, a control system for said valves comprising an exciting electrode in each of said valves, a source of alternating control potential, 2. capacitor connected to said source to be charged to alternate polarity during successive half-cycles of said source, connections from the opposite sides of said capacitor to the respective exciting electrodes, each of said connections including a unidirectional conductor, a return circuit from said exciting electrodes, impedance means connecting said return circuit to both sides of said capacitor and an electric valve in said return circuit for controlling the application of excitins. impulses to said exciting electrodes.

14. An electric current translating system comprising at least a pair of alternately conducting vapor electric valves, a make-alive type exciting electrode in each of said valves, a capacitor, means for charging said capacitor in alternate polarity, an intermediate terminal between said make-alive electrodes, a connection from one side of said capacitor to one of said make-alive electrodes, a connection from the opposite terminal of the capacitor to the other make-alive electrode, a return connection from the intermediate terminal, circuit means including a pair of oppositely disposed unidirectional conductors connecting said return circuit to ,both sides of said capacitor, a controlled electric valve in said return connection for controlling flow of current from said capacitor to said make-alive electrodes and polarity responsive means for determining which of said make-alive electrodes is energized by said current flow.

15. A control system for a pair of alternately conducting vapor electricvalves comprising a make-alive electrode in each of said valves, a source of alternating control potential, capacitor means, connections for cyclically charging said capacitor means in alternate polarity, circuit means for supplying energy from said capacitor to each of said make-alive electrodes, said circuit means including polarity responsive means in series with each of said make-alive electrodes, a winding connected across said capacitor means, an intermediate connection in said Winding, a common return circuit from said make-alive electrodes to said intermediate connection and a single controlled discharge device in said common return circuit for controlling current flow to each of said make-alive electrodes.

16. A control system for a pair of alternately conducting vapor electric valves each of which includes at least a vaporizable cathode, an anode and a make-alive electrode, comprising capacitor means, means for alternately charging said capacitor means in opposite polarity, winding means connected to opposite sides of said capacitor means, an intermediate connection in said winding means, a single controlled unidirectional discharge device for controlling current flow through said winding means and polarity responsive circuit means for impressing the potential of said winding means on said make-alive electrodes.

17. In an electric conversion system having at least a pair of alternately conducting electric valves of the make-alive type, a control system comprising a make-alive electrode in each of said valves, a source of alternating control potential, a capacitor connected to be charged from said source in alternate polarity by alternate halfcycles of said source, a bridge-type rectifier connected across the terminals of said capacitor, the respective make-alive electrodes of said alternately conducting valves connected in series circuit relation in alternately con-ducting arms of said bridge-type rectifier, a grid-controlled electric valve connected across the direct-current terminals of said bridge-type rectifier, and a resistance network energized by the potentials across the capacitor impressing control potential on the grid of said gridcontrol1ed electric valve.

18. In an electric conversion system having at least a pair of alternately conducting electric valves of the make-alive type, a control system comprising a make-alive electrode in each of said valves, a source of alternating control potential, a capacitor connected to be charged from said source in alternate polarity by alternate halfcycles of said source, bridge-type rectifier connected across the terminals of said capacitor, the respective make-alive electrodes of said. alternately conducting valves connected in series circuit relation in alternately conducting arms of said bridge-type rectifier, a grid-controlled electric valve connected across the direct-current terminals of said bridge-type rectifier, a voltage divider connected across the capacitor, a full-wave rectifier fed from said voltage divider, connections impressing the output potential of said fullwave rectifier on the grid of said grid-controlled electric valve, a second voltage divider connected across the capacitor, an intermediate connection in said voltage divider, an impedance connected between the negative terminal of the bridge-connected rectifier and the intermediate connection of said voltage divider and connections :forimpressing a voltage across a portion of said impedance on the grid of said grid-controlled valve.

aaaaene 19. An electric current conversion system com= prising at least a pair of alternately conducting valves each of said valves including a main anode, a vaporizable cathode and a make-alive electrode, a source of alternating control potential, a capacitor connected to be charged from said source, connections iroi n the opposite sides of the capacitor to the respective makealive electrodes, polarity responsive means for controlling current .flow in said connections, oppositely disposed unidirectional conducting devices connected 12 across a said capacitor, a connection from rtheicaiihis. odes of said valves to a point intermediatesaid unidirectional conductors and a controlled elec- N 0 references cited. 

